1. Field of the Invention
Method of measuring soil gas emanation. Particularly a method of measuring both soil gas concentration and emission flux as an aid to oil and gas exploration, minerals exploration, evaluation of polluted sources and the like.
2. Description of the Prior Art
Introduction
The present method of measuring said gas concentrations and emissions enables the control of planning and scheduling of soil gas and atmospheric sampling as a means of geological and geophysical exploration and evaluation, and of detecting and assessing buried natural and anthropogenic substances by reference to natural geophysical phenomena. More specifically, this invention enables an enhanced measuring of soil gas concentrations and emission flux rates, with favorable and unfavorable periods being delineated upon the calculation of the curve of variation of earth tidal forces, and the empirically determined relationship of soil gas concentration and emission flux rate to the tidal forces. The novelty and utility of the invention in a variety of applications is discussed below. For many years various kinds of measurements have been made based on soil gases occurring near or at the surface of the earth and in the atmosphere as a means of detecting and otherwise characterizing source materials buried below the surface and from which such gases arise. Several examples may be cited.
Oil and Gas Exploration
In the oil and gas industry, geological exploration has included detection of hydrocarbons arising from accumulations at depth. In the early days of oil exploration, visible seepage of liquid oil out of the earth enabled detection of deposits near the surface. Later, more sensitive seep detection techniques were employed based on the upward migration of gases arising from volatile components of deeper deposits. Direct Techniques of "micro-seep" detection are based on the hydrocarbons arising from the source deposit itself; indirect techniques are based on non-hydrocarbon gases that may be associated with the deposit, e.g. a radon halo or anomalous concentrations of helium.
Direct techniques have received the most attention and include:
a) microbiological methods based on certain microorganisms that thrive on hydrocarbons that have migrated to the surface (G. G. Soli, "Microorganisms and Geochemical methods of Oil Prospecting", Bulletin of Amer. Assn. of Petroleum Geologists, v. 41, no. 1, pp. 134-40, January 1957; Stravinski, "Microbiological Method of Prospecting for Oil", World Oil, v. 141, no. 6, pp. 104, 106, 109-110, November 1955); and PA1 b) the development of certain carbonates formed upon hydrocarbon exposure that can be detected by controlled thermal cycling and monitoring the evolved carbon dioxide from near-surface soil samples (E. McDermott, U.S. Pat. No. 2,590,113, Geochemical Prospecting).
Methods (a) and (b) involve long term exposures to hydrocarbon gases thereby providing stable averaging. However, these techniques are subject to specific near surface characteristics and to important logistic disadvantages: complex analytical techniques, and resultant delays in obtaining results, important to the conduct of grass roots exploration; and possible non-specificity of the method with respect to hydrocarbon sources. Further, these methods do not identify individual hydrocarbons or their relative proportions.
The most frequently employed and accepted gas tracer methods involve direct measurement of the hydrocarbons in soil gas in order to avoid the difficulties cited above. The gas sample is generally obtained by drawing air from a sealed off short drill hole into a collecting medium, or by aspirating soil gas through a metal probe driven several feet into the ground. The collected samples are then analyzed for specific hydrocarbons associable with a source deposit, e.g. a series of low molecular weight alkanes or alkenes and various aromatic compounds. Parallel applications involve sampling the atmosphere (G. H. Milly, U.S. Pat. No. 3,734,489) or the ocean bottom (G. J. Demaison and I. R. Kaplan, U.S. Pat. No. 4,659,675) into which the hydrocarbons have emanated. It is relevant to the present invention as discussed further below that these more generally employed direct soil gas techniques provide nearly instantaneous snapshots in time of the hydrocarbon concentration although, as we shall show, the concentration typically fluctuates widely over short periods of time; and conventional soil gas surveys pursuant to the usual grid or fence sampling plan are accomplished with temporally sequential measurements. The resultant asynoptic data cannot be validly compared point to point in mapping the soil gas hydrocarbon field because of the varying concentrations over the time span of measurements. The present invention provides a time span sampling protocol that accommodates the natural phenomena responsible for large fluctuations of concentration, and thereby remedies the deficiencies of prior techniques.
Minerals Exploration
In the metals mining industry, tracer gas techniques have been most notably employed in exploration for uranium and for gold. In the case of uranium, the related pathfinder gas is the noble gas radon (Rn-222) arising in the radioactive decay chain of uranium. Although gold itself has no gas phase, it frequently occurs in association with mercury compounds and small amounts of free elemental mercury resulting from biochemical or geochemical reduction of mercury salts. Even though mercury has a low vapor pressure (approximately 0.001 mm Hg), this is sufficient to produce detectable soil gas concentrations.
The variability of soil gas concentration has been widely experienced in uranium exploration--where the techniques have been extensively employed--to the extent that irreproducibility of measurments has led to an attitude of distrust of the technology as other than a supporting but often suspect adjunct to more familiar methods.
Other potential applications of presently employed gas tracer techniques in a variety of other mineral exploration applications here involve considerations similar to those discussed above. These include: mapping phosphate beds based on radon emission from low level uranium content therein; exploration for sulfur deposits in salt domes, based on emission of gaseous sulfur compounds, carbon dioxide, and radon; exploration for geothermal sources; exploration for subsurface water bodies in desert regions based on water vapor emission. All present similar complications with respect to previously unexplained variability in relation to presently employed gas tracer techniques. Even carefully controlled experiments of radon gas concentration over a 13-month period (R. L. Fleischer and A. Mogro-Campero, Geophysical Research Letters, pp. 362-4, May 1979) have led to admitted lack of explanation of the variation and speculations as to in-earth convective cells that our data indicate not to be a correct interpretation.
Evaluation of Pollutant Sources
In the area of toxic materials management, soil gas techniques are applicable in the detection and evaluation of buried substances, either naturally occuring or anthropogenic in nature and not readily detected by other surface techniques.
(i) A prominent example of a naturally occurring hazard is radon emitting into residences, schools and other buildings. Evaluation of large statewide areas to define high and low risk regions can be done using atmospheric sampling techniques to map regional variations in radon emission intensity (G. H. Milly, Mobile Measurement of Radon Concentration in East Coast Terrain, U.S. Environmental Protection Agency Contract No. 68-01-7341, February 1987, Quadrel Research Corporation). As in the case of uranium exploration based on soil gas radon, large variations in emission rate can and do typically occur over a period of several hours. Depending on when the measurements are made, an area actually presenting a substantial threat can appear harmless. Another example of natural pollutants relates to sulfur compounds, and practical concerns of their role in acid rain. Emission rates have been measured employing a large area sampling grid over the eastern United States to assess the fractional contribution of natural sulfur to industrial sulfur dioxide and sulphate loadings of the atmosphere. (D. F. Adams et al., Biogenic Sulfur Emissions in the SURE Region Electric Power Research Institute, EA-1516, Project 856-1, September 1980). However, potentially large temporal variations across the non-simultaneous but serial grid-point measurements were not recognized or accommodated.
(ii) Anthropogenic hazards are represented most pervasively by numerous widespread toxic chemical waste sites where toxic industrial and commercial materials have been dumped and then covered over with earth. Waste site investigations involve various methods of detection and evaluation of the content and area perimeters of subsurface contamination including measurements of volatilized vapors of buried contaminants contained in soil gas. Post-closure monitoring after clean-up may also employ soil gas techniques. Current practice of soil gas mapping entails sequential point to point sampling over an area array. The problems previously cited regarding temporal variation are such that, under certain conditions, misleadingly little or no emission is detectable; and, under favorable conditions for emission, short term variability distorts the true pattern because of asynoptic observations. Similar considerations apply to other anthropogenic hazards such as radon emissions from uranium mill tailing piles.
Summary
In summary, there are numerous examples of economic and public health importance where assessments may be made on the basis of measurements of soil gas either directly, or indirectly through resultant atmospheric concentration; and the utility or even validity of these techniques is readily thwarted by wide variations in emission rates and resultant atmospheric concentration levels. Applicant explains this variation and provides a method for enhanced measuring of soil gas, so as to control planning and scheduling of measurement programs and minimize or eliminate the effect of variability; while providing enhanced comprehension in the analysis of data by recognizing the source of these variations.